In connection with municipal and many industrial water treatment systems, the water or wastewater (referred to herein simply as “water”) needs to be purified. For example, one such system may be a drinking water system where drinking water is produced from surface water and another system may be a municipal wastewater treatment wherein the wastewater needs to be treated so that it can be discharged or reused in industry or for irrigation and similar purposes. In order for such treated water to be useful, pathogens, protozoans, phosphorus and other pollutants need to be removed from the water. More-over, organisms, such as Cryptosporidium and Giardia and their cysts, need to be removed as well.
In such a purification process, the water can be subjected to precipitation and/or flocculation. In this regard, conventional chemical purification can include one or more flocculation tanks in which the water is agitated with stirrers or agitators. Thereafter, the water passes through one or more sedimentation basins after appropriate chemicals have been added. One of the disadvantages of conventional chemical purification processes is the large area required for the flocculation tanks and sedimentation basins. A further disadvantage of conventional chemical purification techniques is the long duration that the water needs to remain in the flocculation tank as well as the sedimentation basin.
The use of flocculation tanks and sedimentation basins alone in the chemical purification process does not typically result in a high enough water purity for many applications. While membrane filtration with a suitably tight membrane can be used to attain a higher level of purification, such membrane filters are expensive and have other disadvantages. On the other hand, a granular media filter, for example, a sand filter, can be added at the end of the purification step to increase the purity of the water being treated. The sand in such sand filters must be cleaned. In some such filters, the sand is cleaned by back-washing it at frequent intervals. In order to avoid shutting down the purification step, it is necessary to provide at least two sand filters, one of which is in use while the other is being back-washed.
The use of two different, separately operated sand filters can be avoided if a continuously operated sand filter of the type disclosed in U.S. Pat. Nos. 4,126,546 and 4,197,201 is utilized. In such a sand filter, the filter bed is continuously cleaned while the filter is in operation. In this regard, the dirtiest sand is taken out of the filter bed, washed and returned to the clean part of the sand bed. In this way, the filter does not have to be taken out of operation for back-washing. A similar type of continuously operating sand filter also is disclosed in U.S. Pat. No. 4,246,102. As disclosed in that patent, the liquid is treated with chemicals before being treated in the sand filter.
In the sand filters of these patents, the liquid is introduced into the lower part of the filter bed. Filtration takes place upwards through the sand bed which is moving downward. In case the sand filter is operated with chemicals being added as disclosed in U.S. Pat. No. 4,246,102, then precipitation/coagulation and/or flocculation occurs during this filtration process. Most of the suspended solids in the feed will be separated near the feed level, which results in the dirtiest sand being in the lower part of the filter. The sand bed is kept in a slow downward motion by an air-lift pump that removes the dirtiest sand from a location close to the bottom of the filter tank. In the air-lift pump, the sand is subjected to a thorough mechanical agitation by the action of the air bubbles within the pump such that the dirt is separated from the grains of sand. The separated dirt is rinsed from the sand in a sand washer near the top of the air-lift pump, the sand washer being disposed concentrically around the upper part of the air-lift pump. The clean sand is returned to the top of the filter bed. Reject water is continuously removed from the sand washer and discharged from the sand filter whereas the filtrate exits from the sand filter as an overflow.
As is indicated in U.S. Pat. No. 4,246,102, the use of such a continuously operating sand filter with chemical treatment makes it possible to reduce the volume of liquid retained in the purification step to about one-tenth of that required for conventional processes. As a result, the area required for that step is reduced and the rate at which liquid passes through the purification step is increased. Moreover, considerably higher purity can be accomplished as compared to the purity attained with conventional techniques using flocculation tanks and sedimentation basins. Advantageously, the particulate filter material is being washed and returned to the filter bed continuously so that the filter material can accept a liquid which is quite dirty and/or contains considerable precipitates without any need for discontinuing the operation of the filter bed for the purpose of back-washing.
In order to further increase the purity level of the water being treated by such sand filters, two continuously operated sand filters can be operated in series with the filtrate exiting the first sand filter being introduced into the feed/input of the second sand filter. Such serial sand filters have been operated successfully in Europe (for example, in Holmsland, Denmark and Lairg, Scotland). However, the amount of reject from those filters and the amount of impurities in the reject makes it difficult and costly to dispose of the reject.
Another example where the sand filters of the type disclosed in U.S. Pat. Nos. 4,126,546, 4,197,201 and 4,246,102 are utilized is the wastewater management system disclosed in U.S. Pat. No. 5,843,308. This system includes two continuously operated sand filters of the type disclosed in U.S. Pat. Nos. 4,126,546 and 4,197,201 with direct filtration of the type disclosed in U.S. Pat. No. 4,246,102. According to U.S. Pat. No. 5,843,308, the sand filters are operated in series in order to eliminate or substantially reduce phosphorus, pathogens and protozoans; for example, Cryptosporidium and Giardia. Unlike the above noted European systems utilizing such sand filters in series, the reject water from the second sand filter is returned to the influent of the first sand filter and the reject water from only the first sand filter is directed to waste. It is the recirculation of the reject from the second sand filter back into the first sand filter that U.S. Pat. No. 5,843,308 asserts solves the problem of separating the above noted pollutants using the known method of operating two continuously operating sand filters in series. However, a system of the type disclosed in U.S. Pat. No. 5,843,308 does not actually provide a solution to the operation of sand filters in series, but instead creates a new and possibly more serious problem. In any such sand filter, the pollutants from the water being treated are concentrated in the reject (possibly in an order of magnitude of 20 times) that is discharged to waste. As a result, the reject from each sand filter has a high level of pollutants and in fact, the pollutants are in a significant concentration level within the reject. In view of the fact that flocculent fragments are difficult to separate from the reject without renewed precipitation and/or flocculation, the internal recirculation of the reject from the second sand filter that contains such pollutants to the input of the first sand filter results in the concentrated pollutants being returned to the first sand filter. This increases, rather than decreases, the chances that the pollutants will be in the treated water as it exits the second sand filter. Further, U.S. Pat. No. 5,843,308 indicates that the reject from the second sand filter is recycled into the first sand filter at a location downstream from the point where coagulants are added to the water that is being treated in the system. Therefore, the recycled reject from the sand filter is not subjected to a renewed coagulation and/or flocculation which would otherwise enhance the chances that pollutants would be separated in the first sand filter. U.S. Pat. No. 5,843,308 also indicates that the preferred coagulant is poly-aluminum-silicate-sulfate (PASS). However, that type of coagulant reacts so quickly that flocculation is essentially immediate. Hence, any coagulation/flocculation carried out in the water that is being supplied to the first sand filter is completed prior to the introduction of the reject from the second sand filter that is being recycled or reintroduced into the first sand filter.
In general, a system like the one disclosed in U.S. Pat. No. 5,843,308 wherein pollutants are separated by a two-step separation device and the pollutants separated in the second step are returned to the first step increases the risk for build-up and breakthrough of pollutants under real life operating conditions. In fact, the reliability of the system proposed in U.S. Pat. No. 5,843,308 is such that it might be necessary to supply a safety system consisting of two additional filter steps in series to those proposed in the patent to ensure that the separation will be reliable.
It is the unique operating method of the continuous backwashing upflow sand filter that allows the use of polymers and coagulants that would quickly clog traditional rapid sand filters. Through this unique capability and the fact that the pressures required to operate the filter are very low, that has resulted in the related patent literature cited in this document. In such filters control is generally limited to pacing the injection of polymer and chemicals in the inflows, and monitoring inflow and product flow rates. However, the prior art does not include improved methods of supervising and controlling such filter systems other than by manual observation and adjustment.